It has been discovered that methods utilizing transient electromagnetic phenomena can detect wall loss, typically but not exclusively due to corrosion, on the conductive walls of pipelines and other types of containers. In the transient electromagnetic (TEM) method, a transmitting antenna and a receiving antenna are placed in proximity to the pipeline wall. The transmitting antenna is provided with an abruptly changing current so as to induce current into the wall of the pipe. The induced current decays over a short period of time; the decay of the induced current is detected by the receiving antenna and a receiver. The TEM method is particularly useful in detecting wall loss due to corrosion on pipelines wrapped with insulating material and covered with a protective metal jacket. Prior art methods such as ultrasonics and radiography have proven inadequate or very expensive in detecting corrosion through insulation and metal jacketing. Insulated pipelines cannot be economically inspected if the insulation must be removed for inspection.
In testing the feasibility of the TEM method of detecting corrosion, the commercially available SIROTEM unit, manufactured by Geoex Pty. Ltd. of Adelaide, Australia was used. The SIROTEM unit is disclosed in Buselli et al., U.S. Pat. No. 4,247,821. The SIROTEM unit was designed with geophysical prospecting in mind. As such, many features of the SIROTEM unit are either unsatisfactory or insufficient for corrosion detection. This is particularly true in noise suppression. Detecting corrosion through insulation and metal cladding requires data acquisition at relatively late times when it becomes difficult to distinguish the signal from noise. By reducing or suppressing noise, the sensitivity of the detection apparatus can be increased. In reducing noise, the apparatus must faithfully amplify and process the time domain signals to prevent distortion of the phase and phase-related parameters of the signals.
Another disadvantage to the SIROTEM unit is its inflexibility in being adaptable to other procedures. The SIROTEM unit is equipped with one transmitting antenna and one receiving antenna. It is difficult to adapt the SIROTEM unit to detection operations on containers, and in particular on pipelines. Pipelines have symmetrical characteristics that can be taken advantage of by detection apparatuses with plural receiving antennas.
Therefore, what is needed is a TEM apparatus with improved noise suppression capabilities and with increased flexibility for use in corrosion detection applications.